1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus that prevents degradation of image quality by a transfer belt.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus is a printing device that prints a black and white or a multicolor image onto a piece of paper by irradiating a photosensitive medium charged to a uniform potential with light to form an electrostatic latent image. The electrostatic latent image is developed with toner using a developing unit. The developed image is transferred and fixed onto the piece of paper.
A conventional electrophotographic image forming apparatus that prints color images includes an exposing unit that emits light corresponding to image information. The exposing unit scans a photosensitive medium to form an electrostatic latent image thereon. Four developing units develop four colors of toner image by supplying toners of cyan (C), magenta (M), yellow (Y), and black (K) color to the electrostatic latent image formed on an outer surface of the photosensitive medium. A transfer belt transfers the toner image from the photosensitive medium to a piece of paper.
FIG. 1 is a cross-sectional view of an intermediate transfer unit of a conventional image forming apparatus. FIG. 2 is a schematic drawing illustrating a failure to transfer a portion of a toner image formed on a transfer belt. FIG. 3 is a plan view of the transfer belt depicted in FIG. 2.
Referring to FIG. 1, the intermediate transfer unit includes a transfer belt 5, an intermediate transfer roller 14, a plurality of supporting rollers 11, 12, and 13, and a nip roller 15. The supporting rollers 11, 12, and 13 support and rotate the transfer belt 5 that is installed around them. The nip roller 15, also mounted inside the loop made by the transfer belt 5, maintains a predetermined nip a between a photosensitive medium and the transfer belt 5.
The transfer belt 5 is mounted to face the photosensitive medium in a span between the intermediate transfer roller 14 and the nip roller 15 so that the toner image developed on the outer surface of the photosensitive medium can be transferred to the transfer belt 5. A transferring bias voltage for transferring the toner image formed on the photosensitive medium to the transfer belt 5 is applied to the intermediate transfer roller 14.
Referring to FIGS. 2 and 3, a protection unit 20 formed to a predetermined thickness is attached to inner and outer surfaces of the transfer belt 5 to protect the transfer belt 5 from damage. When the protection unit 20 is attached to the transfer belt 5, as depicted in FIG. 2, there is an overlapping part 40 where the transfer belt 5 and the protection unit 20 overlap, or a seam part 30 where the transfer belt 5 and the protection unit 20 are joined.
When the seam part 30 is located at a position where a high degree of stress is applied when the image forming apparatus is not in operation, a wrinkle may be formed due to different stretching rates of the transfer belt 5 and the protection unit 20. For example, as depicted in FIG. 2, if the seam part 30 of the transfer belt 5 is located at a supporting roller 11, 12, and 13, a wrinkle can be formed on a boundary face of the seam part 30 due to the different stretching rates. Also, when the overlapping part 40 is located at a position where tension is high, a localized deformation of the transfer belt 5 can occur by the concentration of stress on an edge part 42 that is thinner than other areas of the overlapping part 40, thereby forming a wrinkle on the transfer belt 5. The formation of wrinkles on the transfer belt 5 become more severe when the image forming apparatus is used in hot and humid environments.
When a new printing operation begins with the wrinkled transfer belt 5, a portion of a toner image may not be transferred from the photosensitive medium to the transfer belt 5 during the transfer process.
Although not shown in the drawings, when an electrostatic latent image formed on an outer surface of the photosensitive medium is developed by a developing unit, a strong vibration is generated on a developing roller at the moment a developing roller driving clutch of the developing unit is engaged or disengaged. The vibration is transmitted to the transfer belt 5 through the photosensitive medium. This vibration causes the formation of cross bands on the toner image being transferred from the photosensitive medium to the transfer belt 5.
Also, the toner image is vulnerable to scattering by an electrostatic force of the intermediate transfer roller 14 when transferring the toner image from the photosensitive medium to the transfer belt 5. This results in a blurry or scattered image on paper.
Also, during continuous printing, if the transfer belt 5 begins to wobble, image quality is further deteriorated.
Accordingly, a need exists for an image forming apparatus having an improved transfer belt to prevent poor image quality.